Timing of Anti-uterotrophic Effects of a Single Injection of Progesterone in the Oestrogen-stimulated Uterus: Comparison of the Sheep and Mouse

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Abstract
Progesterone (P) is anti-uterotrophic in the oestradiol (E2)-stimulated uterus of the mouse but not the sheep. To further understand this species difference, the time course of some early effects of a single injection of P on the E2-stimulated uterus has been examined. In the mouse, P caused a prompt (2-4 h) decline in uterine wet weight and in vitro rate of RNA synthesis, and a more gradual decrease (4-12 h) in RNA: DNA and protein: DNA ratios and in vitro rate of protein synthesis in whole uterus. In the sheep, P had no effect at any time on either whole uterus or caruncular endometrium. The results support the concepts that (l) anti-uterotrophic effects of Pin E2-stimulated mice are mediated by a direct P effect on nuclear E2 receptor level, and (2) in the sheep P is not anti-uterotrophic because it does not regulate nuclear E2 receptor level.

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